Safety margin evaluations for transtympanic laser stimulation of the cochlea from the outer ear in Mongolian gerbils (Meriones unguiculatus)

Abstract

AbstractInfrared laser stimulation of the cochlea has been proposed as a possible alternative to conventional auditory prostheses. The safety margin of laser stimulation must be determined for the practical application of the laser to a prosthesis. This study assessed the effect of laser-induced damage to the cochlea on auditory perception. To examine the effects of laser-induced injury of the auditory system on the perception of auditory stimuli, we trained Mongolian gerbils (Meriones unguiculatus) using a head-fixed classical conditioning task. White noise of 80 dB sound pressure level (SPL) was presented as a conditioned stimulus for a reward (a drop of water), and licking behavior was recorded as a conditioned response. After training, an optic fiber was inserted into the ear canal. Each subject was exposed to continuous pulsed laser exposure of 1.6, 3.3, 6.6, 26.4, 52.8, or 105.6 W/cm2for 15 h. White noise of various intensities was presented without pairing it with water before and after laser exposure, to assess the decrease in auditory perception due to laser-induced injury. The licking rate did not change after laser exposure of 6.6 W/cm2or weaker but drastically decreased after 26.4 W/cm2or higher. These results suggest that the injury threshold in Mongolian gerbils for laser stimulation is between 6.6 and 26.4 W/cm2; laser exposure over 6.6 W/cm2could be out of the safety margin. These findings will contribute to the appropriate delimitation of safe and effective laser stimulation parameters in future research.